This invention relates generally to audio amplifiers and, more particularly, to circuits for protecting speakers from damage caused by DC voltage imbalance of the amplifier, high frequency signals, turn-on or turn-off transients, and the like.
The advent of transistor technology has permitted the design of audio amplifiers having speakers which are DC connected to the amplifier output. Amplifiers of this type have many advantages, however, they also have many problems which heretofor have not been solved satisfactorily. For example, amplifier turn-on and turn-off transients, if connected to the speaker, produce an annoying thump that can damage a speaker. In addition, component failure or aging can cause a non-zero quiescent output voltage to develop which produce a direct current through the connected speaker coil which, if sufficiently large, will cause damage to or even destroy it. Some amplifier failures produce high frequency oscillations which may damage both the amplifier and speaker.
In attempting to overcome some of the above mentioned problems, protective circuits have been designed which, on detecting a DC voltage at the amplifier output terminal, shorts the amplifier output to ground thereby by-passing the speaker. This approach, however, can cause a large current to flow in the amplifier output transistors which could cause them to burn out.
Another frequently employed approach is to provide a circuit which disconnects the speaker from the amplifier output if a DC output voltage of sufficient magnitude is present. Known circuits of this type, however, have not provided isolation between different channels of an amplifier system so that, under certain circumstances, it is possible that a failure in more than one channel could occur simultaneously and the protection circuit would fail to protect the speakers on these channels. Additionally, a protection circuit of this type does not prevent either the thumping noise or possible damage resulting from turn-on or turn-off amplifier transients nor does it address the problem of spurious high frequency output signals.